[fmap]

Lenses are more than reified record labels though. There is a hierarchy of concepts that can be freely composed based on which features your data structure actually supports. In particular, lenses can be composed with traversals ("lenses" pointing at multiple fields) yielding "LINQ" like features without introducing new syntax or concepts.

The main problem with lenses is that common lens libraries look extremely complicated at first glance and seem to be solving a very simple problem. That rightfully puts most people off of learning what all the fuss is about.

[mlthoughts2018]

fmap replying to foldr in a subjective argument about functional lenses... what have I done to deserve this hell.